descriptor_utils: check for size overflow in new()

Move the check for length overflow that was in available_bytes() into
Reader::new() and Writer::new().  This simplifies callers, since they
can assume that once a valid Reader or Writer has been constructed,
available_bytes() cannot fail.  Since we are walking the descriptor
chain during new() anyway, this extra check should be essentially free.

BUG=None
TEST=cargo test -p devices descriptor_utils

Change-Id: Ibeb1defd3728e7b71356650094b0885f3419ed47
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1873142
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen Barber <smbarber@chromium.org>

1 files changed, 2 insertions, 6 deletions

diff --git a/devices/src/virtio/input/mod.rs b/devices/src/virtio/input/mod.rs
index 2694d46..706a563 100644
--- a/devices/src/virtio/input/mod.rs
+++ b/devices/src/virtio/input/mod.rs
@@ -387,9 +387,7 @@ impl<T: EventSource> Worker<T> {
     ) -> Result<usize> {
         let mut writer = Writer::new(mem, avail_desc).map_err(InputError::Descriptor)?;
 
-        while writer.available_bytes().map_err(InputError::Descriptor)?
-            >= virtio_input_event::EVENT_SIZE
-        {
+        while writer.available_bytes() >= virtio_input_event::EVENT_SIZE {
             if let Some(evt) = event_source.pop_available_event() {
                 writer.write_obj(evt).map_err(InputError::WriteQueue)?;
             } else {
@@ -445,9 +443,7 @@ impl<T: EventSource> Worker<T> {
         mem: &GuestMemory,
     ) -> Result<usize> {
         let mut reader = Reader::new(mem, avail_desc).map_err(InputError::Descriptor)?;
-        while reader.available_bytes().map_err(InputError::Descriptor)?
-            >= virtio_input_event::EVENT_SIZE
-        {
+        while reader.available_bytes() >= virtio_input_event::EVENT_SIZE {
             let evt: virtio_input_event = reader.read_obj().map_err(InputError::ReadQueue)?;
             event_source.send_event(&evt)?;
         }